1. Field of the Invention
The present invention relates to a boundary layer microphone also called an interfacial microphone.
2. Description of the Related Art
Boundary layer microphones have been known for some considerable time. One example is the MKE 212 P microphone type made by Sennheiser electronic GmbH & Co. KG, Germany. The latter is a permanently polarized condenser microphone for inconspicuous mounting in a wall, on the floor or on a table. Stereo recordings using such a boundary layer microphone are especially clear, creating a spatial impression of unusual breadth. The known boundary layer microphone MKE 212 P displays omnidirectional characteristics and a frequency range of 20 to 20,000 Hz.
However, the polar response of such boundary layer microphones is the same for every direction of incident sound. In certain applications, e.g., recording speech or music, and particularly in conference equipment, it is preferable to focus the sensitivity of the microphone in the direction of a speaker or musician. It is then necessary to filter out secondary noises, reverberations as well as other speakers or musicians. At the same time, however, the known benefits of boundary layer microphones are to be utilized. Mounting these microphones on large boundary surfaces, such as floors, walls, tables, lecterns, or similar furnishings or appliances with large surfaces prevents the unwanted comb filter effects that are caused by reflections from large nearby boundaries such as the floor or a table. The comb filter results in extreme distortion of the frequency response curve, which then undulates considerably, with deep notches.
Boundary layer microphones with directional characteristics, as known, for example, from the paper entitled "BOUNDARY-LAYER MICROPHONES WITH DIRECTIONAL CHARACTERISTICS" by Beckmann, "AES 75th Convention 1984 March 27-30, Paris", are used more and more often in conferencing facilities or in film and television production because they can be mounted very inconspicuously. In most cases, electret condenser microphone capsules with cardioid or super-cardioid characteristics are used, which are mounted above or inside a flat surface. These capsules mostly have a very weak bass response, a high level of inherent self-noise and a very unfavorable dependence of frequency response on directional characteristics. It declines rapidly at frequencies above 2,000 Hz or so, is lost entirely in many cases, or the microphone is even more sensitive from behind than from the front in some parts of the frequency range. This is due to precisely the type of mounting in front of or in a boundary surface, which is not entirely without its specific problems. Reflections between the microphone and the boundary may be generated, as may resonance effects in the cavities in which the sound must be "diverted" to the front or rear sound inlets of the microphone capsules.